Change FanoutChannel to synchronously process handle destruction.
Change FanoutChannel to synchronously process handle destruction.

Summary: This ensures that closing a FanoutChannel will synchronously cancel its input receiver. We don't need to run this code on the provided executor, because it is not running as part of a callback (and because we already have a lock).

Reviewed By: aary

Differential Revision: D32191795

fbshipit-source-id: 9ec2a2303a12580e052de7af8651ebf5b773b8b6

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly